Myositis is one form of inflammatory myopathy involving chronic inflammation of the skeletal muscles. It is a rare disease in which the immune system chronically inflames the body's muscle tissue. The etiology of the immune system's attack in currently unknown. Persistent inflammation progressively weakens the muscles, causing muscle soreness, joint pain and fatigue.
Myositis can take several forms and usually develops slowly over time and can range in severity from mild to debilitating or worse. The three major types of myositis are dermatomyositis (DM), polymyositis (PM), and inclusion body myositis (IBM). PM affects skeletal muscles on both sides of the body and mostly affects people between the ages of 31 and 60. Progressive muscle weakness often leads to difficulty swallowing (dysphagia), rising from a sitting position, climbing stairs, lifting objects, or reaching overhead. People with PM may also experience arthritis, shortness of breath, and heart arrhythmias.
In addition to progressive muscle weakness, patients with DM have a characteristic skin rash that precedes or accompanies progressive muscle weakness. The rash looks patchy, with purple or red discolorations, and is characteristically found on the eyelids and on skin overlying joints of knuckles, elbows, knees, and toes. Red rashes are also found on other locations such as the face, neck, shoulders, upper chest, and back. The rash sometimes occurs without obvious muscle involvement.
IBM is characterized by gradual (over months or years) progressive muscle weakness and wasting that affect both proximal and distal muscles. IBM affects individuals in a variety of different ways from the age of onset. Symptoms of the disease usually begin after the age of 50, although the disease can occur earlier. Muscle weakness may affect only one side of the body. Falling and tripping are usually the first noticeable symptoms of IBM. For some individuals the disorder begins with weakness in the wrists and fingers that causes difficulty with pinching, buttoning, and gripping objects. There may be weakness of the wrist and finger muscles and atrophy (thinning or loss of muscle bulk) of the forearm muscles and quadriceps muscles in the legs. Difficulty swallowing occurs in approximately half of IBM cases. Unlike PM and DM, IBM occurs more frequently in men than in women. In addition to physically symptomatic changes, the muscles of IBM patients have small degenerative structures that with special staining appear as holes called vacuoles in the affected muscle fibers.
Diagnosing IBM can be challenging. Circulating autoantibodies, such as anti-Jo and anti-SRP, have been identified in PM and DM, and have been used to aid both in diagnosis and the treatment of these two conditions. However, to date, no autoantibody has been reported to be associated with IBM. Because IBM affects different people in different ways and at different rates, there is no “textbook case” of IBM. Consequently, IBM is often initially misdiagnosed and a definitive diagnosis is delayed. IBM is often misdiagnosed as some other inflammatory myopathy, usually PM. For example, a course of prednisone is typically completed with no improvement and eventually IBM is suspected. IBM weakness comes on over months or years and progresses steadily, whereas PM has an onset of weeks or months. Nowadays, differential diagnosis of IBM involves systematically ruling out other possible causes such PM, DM, chronic inflammatory demyelinating polyradiculoneuropathy, Duchenne muscular dystrophy, and myasthenia gravis etc. Currently the only definitive test for IBM is a muscle biopsy to confirm the physical changes in the affected muscles.